WASHINGTON—Women who serve on board U.S. Navy ships and young service members at large training bases were at the greatest risk of becoming victims of sexual assault, according to a study commissioned by the Pentagon and released Friday.

The study, which examined the persistent problem of sexual assault in the military, found that women serving at the U.S. naval base in Charleston were at highest risk nationwide, with 17%—or nearly one in six—reporting in a survey that they had been sexually assaulted.

The Pentagon commissioned Rand Corp., a research institution, to conduct a large-scale study in 2014. Rand surveyed approximately 150,000 service members across the various military branches and at bases and installations world-wide.

Maj. Carla Gleason, a Pentagon spokeswoman, said the data is from several years ago and the military has undertaken strenuous efforts over a decade to address the problem. The survey itself reflects a commitment to fixing this problem, she said.

“The challenge before us now is to identify things we can change about the local environment in tandem with our ongoing efforts to reduce sexual assault,” she said.

The incidence of sexual assault reported in the study is based on the 2014 survey results, not criminal complaints or legal action. Based on those results, the Charleston Navy base in South Carolina posed the highest risk to Navy women, followed by U.S. ships, including major aircraft carriers.

For Army women, the highest risk locations included Fort Huachuca, Ariz., Osan Air Base in South Korea; and Fort Drum, New York. For Air Force women, the highest risk locations are Vance Air Force Base and Altus Air Force Base, both in Oklahoma, and Laughlin Air Force Base in Texas.

On the largest installations, the survey found more than 500 reported assaults. Overall in 2014, some 20,300 active service members said in the survey that they were assaulted and 116,000 said they were sexually harassed.

The study found that troops assigned to medical facilities had a lower risk of sexual assault.

The report issued Friday had been delayed for several months as defense officials sought to edit it. A Rand spokesman, Jeffrey Hiday, said the changes were sought “for clarity and to highlight the limitations of the study,” and Rand made no substantive changes.

The Pentagon commissioned the study in 2014, as it began to try to further quantify and crack down on sexual assault and harassment across the military services, though it remains an acute problem for the Pentagon and lawmakers.

“There continues to be enormous frustration among members about how difficult it’s proving to end this,” said a congressional staffer familiar with the matter. “It’s not necessarily a frustration with what DoD is doing; it’s just a really tough thing to fix.”

Because of the prevalence of assaults on large bases, Rand recommends directed training to focus efforts on those locations.

The Rand survey had gaps in its information collection, including a lack of full access to Marine Corps installations, but the findings still have relevance, according to researchers and officials.

Sexual assault in the military rose to prominence in the wake of the Tailhook Association conference in Las Vegas in 1991, where some 100 U.S. Navy and Marine Corps aviators allegedly sexually assaulted dozens of men and women. As a result, some top Navy officials saw their careers end abruptly. In the mid-2000s the Pentagon began surveying troops on the matter and working to tackle the problem.

Since then, the number of those reporting assaults has risen, which the Pentagon says indicates more troops are willing to talk about the problem.

The Rand report also showed a high correlation between instances of sexual harassment and sexual assault, though the number of reported harassment incidents is higher than the number of assaults.